This invention relates to method and apparatus for analyzing elemental composition of furnace off-gasses, and more particularly to method and apparatus for continuous, real time monitoring of the elemental content of furnace off-gasses using a microwave-induced plasma and atomic emission spectroscopy which could be implemented in situ.
In the global effort to protect the environment, there exists the need to monitor exhaust gases from all types of furnaces to detect the emission of hazardous chemicals. It is desirable that such a monitoring device be capable of continuous, real time application under harsh and variable conditions. In particular, location in situ or close proximity in the furnace off-gas stream would facilitate accurate spatially resolved measurements. It is also desired that such a monitoring device be able readily to detect the presence of hazardous elements, such as trace metals including lead, mercury, arsenic, beryllium, chromium, antimony, barium, cadmium, thallium, nickel, selenium, and silver as soon as they appear in the exhaust gas and at levels below Environmental Protection Agency mandated threshold levels. Such a device must also be useable in or in close proximity to the hot, dirty environment of a furnace and be capable of detecting the presence of many different elements simultaneously.
Instrumentation for the sensitive analysis of materials developed to date suffer from limitations including their not being true continuous, real time devices, not applicable to in situ measurements, not capable of detecting trace metals, or being limited to monitoring only one element at a time. The use of plasma sources for elemental excitation or detection is currently the primary means for detection of trace elements in solids, liquids and gasses. M. W. Blades, P. Banks, C. Gill, D. Huang, C. Le Blanc, and D. Liang, Application of Weakly Ionized Plasmas for Materials Sampling and Analysis, IEEE Trans. on Plasma Sci., Vol. 19, pp. 1090-1113, 1991 have reviewed such technology, which included conductively coupled plasmas, microwave-induced plasmas, and other techniques. None of the techniques so described are applicable to continuous, real time in situ measurements. Fast Fourier transform spectroscopy, as described by J. Demirgian, Continuous Monitor for Incinerators, U.S. Department of Energy Information Exchange Meeting on the Characterization, Monitoring, and Sensor Technologies, Dallas, TX, June 3-4, 1992 can be used for continuous, near real time monitoring of molecular gases, but is not capable of the detection of metals, especially if the metals are in particulate form. Commercial in situ detectors, such as the Bacharach Instrument Company mercury sniffer model MV-2J-W and the Pacific Northwest Laboratory Halo-sniff spectrochemical emission sensor are limited to detecting only one element at a time. Further, these devices cannot be used for continuous, real time measurements of metals in a wide range of particulate as well as vapor form in a furnace environment. They must pull in a gas sample through a pipe and/or small orifice, which if used in a hot exhaust environment could give false results due to condensation or blockage due to larger particulates.
F. C. Fehsenfeld, K. M. Evenson, and H. P. Broida, Microwave Discharge Cavities Operating at 2450 MHz, Rev. of Sci. Instrm., Vol. 36, pp. 294-298, (1965) described a number of microwave-induced plasma (MIP) resonator cavity structures. One such structure had a built in taper to reduce its height to increase the electric field strength for plasma breakdown. This device was a resonator, not a shorted wave guide. It also included a number of features that limited maximum microwave power, such as a cable connection to the source of such power. None of the devices described by Fehsenfeld can be used in a furnace environment.
R. M. Barnes and E. E. Reszke, Design Concepts for Strip-Line Microwave Spectrochemical Sources, Anal. Chem., Vol. 62, pp. 2650-2654, (1990) described a shorted strip-line microwave MIP arrangement with a dielectric tube through the device one-quarter wavelength from the shorted end. Again, the features of this device, such as the presence of the strip-line and the cable connection to the source, would limit the maximum power operation of this device. Furthermore, this device could not be used in the hot, dirty furnace exhaust environment.
H. Matusiewicz, A Novel Microwave Plasma Cavity Assembly for Atomic Emission Spectrometry, Spectrachimica Acta, Vol. 47B, pp. 1221-1227, (1992); Y. Okamoto, Annular-Shaped Microwave-Induced Nitrogen Plasma at Atmospheric Pressure for Emission Spectrometry of Solutions, Analytical Science, Vol. 7, pp. 283-288, (1991); and D.K. Smith and D.L. Smatlak, Microwave Atmospheric Pressure Plasma Torch, Characteristics and Application, 27th Microwave Symposium, Washington, D.C., Aug. 2-5, 1992 described higher power MIP devices connected to the microwave source directly by wave guide. These devices utilize dielectric tubes and lack the provision for remote calibration, ignition or heating, which make them unsuitable for use in a furnace environment.
Other microwave-induced plasma-atomic emission spectroscopy devices are described by K. A. Forbes, E. E. Reszle, P. C. Uden, and R. M. Barnes, Comparison of Microwave-Induced Plasma Sources, J. of Analytical Atomic Spectrometry, Vol. 6, pp. 57-71, 1991; J. P. Matousek, B. J. Orr, and M. Selby, Microwave-Induced Plasmas: Implementation and Application, Prog. Analyt. Atom. Spectrosc., Vol. 7, pp. 275-314, 1984; S.R. Goode and K. W. Baughman, A Review of Instrumentation Used to Generate Microwave-Induced Plasmas, Applied Spectrosc., Vol. 38, pp. 755-763, 1984; and A. T. Zander and G. M. Hieftje, Microwave-Supported Discharges, Applied Spectrosc., Vol. 35, pp. 357-371, 1981.
Real time, remote calibration is also an important feature for any continuous, real time MIP device which must operate over a variable range of gas flow composition. A furnace off-gas stream will consist of the main working gas, such as air or nitrogen, along with a variable and not-well characterized waste off-gas. Atomic emission line intensities depend, in part, on the plasma gas mixture (matrix effect). This makes in situ calibration necessary for quantitative measurements. The use of laser ablation to introduce samples for calibration purposes into an MIP device has been described by T. Ishizuka and Y. Uwamino, Atomic Emission Spectrometry of Solid Samples with Laser Vaporization-Microwave Induced Plasma System, Anal. Chem., Vol. 52, pp. 125-129, (1980). However, this device would not work for remote calibration because an absolute calibration is required, not the relative one for which the Ishizuka et al. device was designed. In the Ishizuka et al. device, tubing between the laser ablation plate and the plasma causes much of the laser sputtered material to condense out making an absolute calibration unreliable.